Hi Richard,
Thanks for your kind reply. It's already great to see where the problem comes from. :)
This video element is not part of any additional CSS we have added, so I guess it is a bug. It's also weird as even when I change the opacity to 0 in the CSS, the videos appear in the preview, but not anymore if you load them on the regular browser. (When I go back to "Customize", the videos are disappeared again, even though the code is still there.)
I will try to fix it with a friend on the weekend who is more advanced than me with CSS and JavaScript, but if you have any idea how to make it work, of course, it would be a great help! ^^
All the best, and stay safe!
Luca